1. Field of the Invention
The invention relates to a fuel cell system, and to a vehicle equipped with the fuel cell system.
2. Description of the Related Art
Practical application of a fuel cell that supplies hydrogen as a fuel gas to a fuel electrode, and that supplies air as an oxidant gas to an oxidant electrode, and that generates electricity through an electrochemical reaction between hydrogen and oxygen in the air while producing water on an oxidant electrode is now being considered.
In such a fuel cell, if at the time of start of operation, the pressure of hydrogen supplied to the fuel electrode and the pressure of air supplied to the oxidant electrode are about equal to the respective pressures occurring during ordinary operation, it sometimes happens that hydrogen gas and air are unevenly distributed in the fuel electrode and the oxidant electrode, respectively, and the electrodes are degraded by electrochemical reaction caused by the uneven distribution of these gases. Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method of preventing the degradation of the electrodes of a fuel cell by causing the pressures of hydrogen and air supplied to the fuel electrode and the oxidant electrode, respectively, at the time of start of operation of the fuel cell to be higher than the ordinary supplied pressures of these gases.
However, if hydrogen gas and air are supplied at high pressure to a fuel cell when the fuel cell starts operation, it sometimes happen that the rate of rise of the voltage of the fuel cell becomes large so that the voltage of the fuel cell overshoots its upper-limit voltage. In conjunction with this problem, Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method in which when hydrogen gas and air are supplied, at the time of starting a fuel cell, at pressures that are higher than their pressures given during ordinary power generation, output electric power is extracted from the fuel cell, and is put out to a vehicle driving motor, resistors, etc., provided that the voltage of the fuel cell reaches a predetermined voltage that is lower than the upper-limit voltage.
In an electric vehicle equipped with a fuel cell, an output electric power command value for the fuel cell is calculated on the basis of a requested electric power from a load system, and the output current-voltage characteristic of the fuel cell. However, at the time of starting the fuel cell, while the voltage of the fuel cell is rising from a starting voltage, electric current does not flow from the fuel cell due to the blocking by a blocking diode. On another hand, for example, in a method in which after the voltage of the fuel cell, at the time of starting the fuel cell, is temporarily raised to an open-circuit voltage (hereinafter, referred to also as “OCV”), a control voltage of the fuel cell is lowered and electric power is extracted from the fuel cell, a control in which the voltage of the fuel cell is set at the OCV so that current does not flow out from the fuel cell until the electricity generation of the fuel cell is permitted. In this case, however, durability of the fuel cell is sometimes impaired.
Therefore, it is desirable that the starting voltage of the fuel cell be promptly dropped from the OCV to a predetermined operation voltage. However, if the total cell voltage of a fuel cell stack is always dropped at low speed for fear of excessive electricity generation of the fuel cell at the time of starting the fuel cell, a very long time is required for the drop of the voltage from the OCV to the predetermined operation voltage. Furthermore, if the total cell voltage is dropped at low speed in the case where the initial total cell voltage of the fuel cell is higher than the voltage in an ordinary operation state, an upper-limit guard for the command voltage that is commanded to the fuel cell cannot be provided in time, so that the total cell voltage of the fuel cell becomes higher than the predetermined operation voltage, and the total cell voltage of the fuel cell stack overshoots, for example, as shown in FIG. 5.
Besides, in the case where the fuel cell generates more electric power than requested by the load system, the surplus amount of electric power is charged into a secondary cell that is provided in the electric vehicle. If the secondary cell is charged in a manner of, for example, high-rate charging, charging from a high SOC (state of charge), charging at low temperature (e.g., 0° C.) (charging in a high resistance state), etc., the secondary cell sometimes becomes overcharged, so that durability of the secondary cell may be impaired.